Canberra, May 25 : A new study has determined that there has been a decline in autumn rain in south-eastern Australia, a major reason of which is changing weather systems originating from the subtropical Indian Ocean.
The study, carried out by Dr Wenju Cai and Tim Cowan from CSIRO, determines that since 1950, Victoria has suffered a 40 per cent decline in autumn rainfall (March to May) compared to the average recorded between 1961-90.
According to Dr Cai and Cowan, the decline has been most prominent in May, which accounts for about half of the total seasonal reduction.
"Through April and May, large increases in sea surface temperatures in the region are usually associated with a transition from an El Nino to a La Nina event," said Dr Cai.
The identified causes show imprints of climate change influences, in part through a reduction in the number of La Nina events, and in part through changing weather systems originating from the subtropical Indian Ocean that are conducive to late autumn rainfall across Victoria.
The researchers found that since 1950 the spatially alternating high and low pressure systems (called pressure wave-trains) conducive to rainfall over southern Victoria in May have been weakening, leading to rising sea level atmospheric pressure over south-east Australia.
"This weakening is reinforced by a warming of the Indian Ocean, which is at least in part due to global warming," said Dr Cai. "This suggests that a component of climate change is active in southern Victoria receiving less rainfall," he added.
Influences from the Indian Ocean sector occur in conjunction with those from the Indonesian Throughflow region, to the north of Australia.
According to Dr Cai, higher sea surface temperatures in the Throughflow region are conducive to rainfall in central and northern south-east Australia, through the familiar tropical northwest cloud bands, which deliver rainfall to the region.
Victoria is not alone among states experiencing rainfall declines.
During the past 50 years, there has been a decreasing trend in rainfall over much of Australia. In south-west Western Australia, the trend is strongest in winter; and in southern Queensland strongest in summer.